The present invention relates to fluorescent lamp phosphors, and particularly relates to a method of preparing a manganese and lead coactivated calcium metasilicate phosphor and the phosphor produced thereby.
Manganese and lead coactivated calcium silicate phosphors emitting in the red region of the visible spectrum are used, for example, in fluorescent lamps and cathode ray tubes. The phosphor, however, is difficult to manufacture because the Ca-Si-Mn-Pb combination is a slowly reacting system. Such reactivity depends on efficient and complete incorporation of the dopant concentration levels of the manganese and lead ions used in the phosphor. Some processes have used an excess quantity of silica in the starting formulation to assist reactivity. However, unreacted components in a phosphor can be detrimental to the light output of a lamp, although an excess amount of unreacted silica is less detrimental than excess metal compounds because of the metal oxide's greater absorption of ultraviolet radiation. Other processes use additives, for example, metal halides to facilitate a complete reaction. Using additives to assist the reaction, however, can increase the particle size of the product under extended firing conditions and can lead to undesirable loss of lead to the environment.
In another context, prior art processes for making calcium silicate phosphors, for example those using silicic acid as a silicon source, produce phosphors of excessively large primary (pre-milling) particle size. such phosphors require extensive milling to achieve the desired final particle size. However, such extensive milling alters the surface characteristics of the phosphor crystals, decreasing the brightness of the phosphor.
In yet another context, prior art processes have used lead halide or lead oxide compounds as the lead source in a vapor state reaction with the CaSiO.sub.3 :Mn. Alternatively, lead fluoride has been added as a powder to the starting formulation for a solid state reaction. However, use of the simple lead halide or oxide compounds requires extensive precautions to prevent release of lead into the environment during the phosphor synthesis process.
It would be desirable to have a red emitting manganese and lead activated calcium silicate phosphor exhibiting a particle size of less than 5 .mu.m without significant loss of lamp brightness. Also desirable would be a process that eases the environmental precautions required during the firing procedure. The phosphor and process described herein were developed to address these needs.